Circuit for energizing an electromagnet



MarchvZ,l 1969` H. s. SCHWARTZ CIRCUIT FOR ENERGIZING -AN ELECTROMAGNET`Filed May 20, 1966 T Lql.

sheet @f2 1 ATTORNEY United States Patent Ogce U.S. Cl. 317-1485 3Claims ABSTRACT F THE DISCLOSURE A single-shot multivibrator circuitemploying a silicon control rectifier for actuating an electromagnet inwhlch a potentiometer is used to vary the time constant in the circuit.

This invention relates to a circuit for energizing an electromagnet and,more particularly, to an improved silicon control rectilier circuit fortiring a solenoid actuated Ihammer in a high speed data printer.

Certain high speed impact printers known in the prior art employ anumber of electromagnetically operated print hammers in combination withtype faces which are mounted on a continuously circulating belt or drum.A tone Wheel or other similar device produces an output signalconsisting of a series of pulses indicative of the position of the typefaces relative to the hammers. This output signal is coupled as oneinput to electronic circuit, the other input to which is 1an input datasignal from a computer, for example. The electronic circuit produces anoutput pulse to ire a hammer when the type face opposite that hammer hasthe character specified to be printed by the input data.

In printing, each hammer must be rapidly accelerated from a restposition and rapidly returned to its rest position in order that thetype face be correctly positioned `and essentially stationary during theperiod of impact. To achieve the necessary acceleration and impact forcenecessary for printing, high energy (150 watts, for example) pulses arerequired for energizing the hammer solenoid. In addition, to achieveprinting that is uniform in appearance, the duration and total energy ofthe drive pulses must be accurately controlled.

A number of circuits have been proposed in the prior art for liringelectromagnetically operated printer hammers. However, these circuitshave proved to be expensive in their construction or inetlicient intheir operation or require elaborately regulated power supplies or acombination of these disadvantages.

IOne object of this invention is the provision of an improved ci-rcuitfor -iiring printer hammers, which is less expensive to construct thanthose of the prior art.

Another object of this invention is to provide an improved circuit whichoperates efficiently and at a relatively high voltage as compared toprinter hammer firing circuits of the prior art.

A further object of this invention is to provide a printer hammer firingcircuit, which can operate from relatively inexpensive power supplywhich supplies the tiring circuits of all the hammers.

3,435,299 Patented Mar. 25, 1969 `One more object of this invention isthe provision of an improved printer hammer tiring circuit whichprovides a current pulse with a short rise time and a predetermined,adjustable magnitude.

Briefly, this invention contemplates the provision of a hammer tiringcircuit in which the anodes of a pair of silicon controlled rectiiiers(SCRs) are cross coupled by a commutating capacitor to form a monostablemultivibrator. Hammer ring pulses are coupled to the gate electrode ofone (first) SCR and a Xed reference potential, which is common to allthe hammer firing circuits of the printer, is coupled to the gate of theother (second) SCR.

The hammer solenoid is coupled in series between the positive terminalof a low impedance power supply and the anode of the first SCR; a timingcapacitor is coupled in series between the cathode of the second SCR andthe negative side ofthe power supply.

In the stable state of the circuit, both SCRs are in a high impedancestate. Following a tiring pulse, the rst SCR switches to its lowimpedance state. The timing capacitor simultaneously commences todischarge at a predetermined, adjustable rate through a resistor and therst SCR. When the cathode potential of the second SOR drops below thereference potential, it switches to its low impedance state and therebyeffectively couples the cornmutating capacitor across the rst SOR, backbiasing it and switching it to its high impedance state. Thereafter, thesecond SCR switches to its high impedance state since the current flowthrough it rapidly falls below the value necessary to sustain conductionowing to the series coupled timing capacitor. The circuit is thusreturned to its stable state.

Having briefly described this invention, it will be described in greaterdetail along with other objects and advantages in the following detaileddescription of a preferred embodiment which may be best understood byreference in the accompanying drawings which form part of the instantspecification and which are to be read in conjunction therewith andwhich like reference numerals are used to indicate like parts in the|various views;

FIG. l is partially a schematic diagram of one embodiment of an improvedhammer -iiring monostable multi vibrator constructed in accordance withthis invention, and is partially a block diagram showing certaincomponents of a high speed printer in which the monostable multivibratorshown schematically is used; and

FIG. 2 is a timing diagram of typical, idealized wave forms for thefiring circuit shown schematically in FIG. 1.

Referring now to FIG. 1, a high speed impact printer typical of thoseknown in the prior art has a row of pivotally mounted hammers 12disposed in a line adjacent a chain formed of type slugs 14 secured to abelt 16 in a manner known in the art. A pair of drive sprockets orpulleys 22 continuously circulate the belt 16 in a countercloclcwisedirection, for example, and a tone Wheel 24, or other suitable apparatus'known in the art, coupled to one of the pulleys produces a series ofoutput pulses which are indicative of the position of the slugs 14relative to the hammers 12. These output pulses are coupled as one inputtoy a decoding and comparing circuit 20. An input from a computer ormagnetic tape transport or the like is coupled to another input terminal21 of the circuit 20.

As will be appreciated readily by those skilled in the art, the threehammers shown are merely representative. A typical printer actuallyemploys from 60 to 120 hammers. Additionally, it will be understood thatthis invention is equally applic-able to printers employing acontinuously rotating drum as to printers employing a chain or belt asshown.

An electromagnet 26 is disposed to attract an armature secured to theshaft of the hammer 12 and to thereby cause the head of the hammer tostrike a web of paper (not shown) passing between the hammers and theslugs 14 and to push the web against one of the type slugs. Eachelectromagnet comprises a U-shaped core of low permeable material suchas iron and a coil 32 made of a large num'ber of turns of low resistancewire wrapped around the base of the core.

yIn accordance with this invention, the solenoid 32 for each hammer 12is energized by a respective SCR monostable multivibrator circuitgenerally indicated by the reference numeral 34. All of the monostablemultivibrators are coupled to a common low impedance direct current(D.C.) power supply 36, and to a common D.C. reference potential source38. Since all of the multivibrator circuits are identical, only thecircuit coupled to the uppermost hammer in FIG. l is shown in detail.

Each multivibrator circuit comprises a pair of silicon controlledrectiers 42 and 44 whose anodes are interconnected by a commutatingcapacitor 46. The coil 32 is connected in series between the positiveterminal of the power supply 36 and the anode of SCR 44. It should benoted that the D.C. impedance in series with the coil 32 determines-both the rise time of a current pulse in coil 32 (the larger the seriesresistance the smaller the rise time) and, for a given power supplypotential, the current flow through the coil (the larger the resistance,the smaller current ow). It is desirable, of course, that the rise timebe short and the current ow large. It has been found advantageous tooperate with series resistance of approximately 50 ohms in mostapplications and a relatively high source voltage. The coil 32 isadvantageously made so that it has as low a distributed resistance as ispractical in order that the amount of heat which must be dissipated fromthe coil be as small as possible. For this reason, preferably, anexternal resistor 48 is coupled in series with the coil to provide thedesired operating characteristic.

The gate electrode of SCR 44 is coupled through a pulse differentiatingnetwork (resistor 52 and capacitor 54) to a terminal 56 to which iscoupled one output terminal of circuit 20. Positive pulses coupled toterminal 56 cause SCR 44 to switch from its high impedance to its lowimpedance state.

A resistor 58 couples the anode of SCR 42 to the positive terminal ofthe power supply 36; a timing capacitor 62 couples its cathode to thenegative or ground terminal of the power supply 36. A forwardly biaseddiode 64 couples the gate electrode of SCR 42 to a positive terminal ofthe D C. reference potential 38, and the gate to cathode bias for SCR 42develops across a resistor 66 which interconnects these electrodes.

Resistor 68 and potentiometer 72 couple the cathode of SCR 42 to theanode of SCR 44 so that, when SCR 44 is in its low impedance state,resistor 68 and potentiometer 72 form a discharge path lfor the timingcapacitor 62. As will be obvious to those skilled in the art, the timeconstant of the R-C circuit comprising resistor 68 and potentiometer 72and capacitor 62 may be varied by varying the potentiometer 72.

Referring now to FIG. 2 in addition to FIG. l, in operation SCR 42 andSCR 44 are both initially in their high impedance (non-conducting) stateso that the potential at 'the anode of each is approximately equal tothe potential at the positive terminal of power supply 36. In responseto a trigger pulse a at terminal 56, SCR 44 switches to its lowimpedance state and its anode rapidly drops to apy proximately groundpotential. Current flow through coil 32 rises rapidly, energizing thesolenoid and thereby ring the hammer. Current also flows from powersupply 36, through resistor 58, commutating capacitor 46 and SCR 44,charging capacitor 46 luntil the potential on the lefthand side of thecapacitor equals the potential at the positive terminal of supply 36.

Timing capacitor 62, which had been charged to the potential of powersupply 36, commences to discharge through resistor 68, potentiometer 72and SCR 44. When the potential of the cathode of SCR 42 drops slightlybelow the reference potential 38, SCR 42 switches to its low impedancestate, eifectively coupling commutating capacitor 46 across SCR 44,thereby back biasing SCR 44 and switching it to its high impedancestate. Following the termination of conduction through SCR 44, the valueof current flow through SCR 42 rapidly falls below the value needed tosustain conduction owing to the series coupled timing capacitor 62, andSCR 42 switches to its high impedance state.

Current flow through SCR 42 prior to its switching to its high impedancestate charges capacitor 62 to approximately the potential of powersupply 36, thus returning to its initial stable state. The circuit willremain in this state until another positive pulse is applied to terminal56.

It should be noted that SCR 42 may not switch to its high impedancestate if the leakage current through capacitor 62 is larger than thatnecessary to sustain conduction in SCR 42. However, this does not affectthe overall operation of the circuit since SCR 42 will be switched toits high impedance state in any event following the switching of SCR 44to its low impedance state owing to the commutating action of capacitor46 following the switching of SCR 44.

It will be understood that certain features` and subcombinations are ofutility and may 'be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims. It is further obvious that various changes may be made indetails within the scope of the claims without departing from the spiritof the invention. It is, therefore, to be understood that this inventionis not to be limited to the specific deals shown and described.

Having thus described the invention, what is claimed is:

1. A monostable multivibrator circuit for energizing an electromagnetcomprising in combination:

a first silicon control rectifier (SCR) having an anode,

a cathode and a gate electrode, and |having a rst state in which theimpedance between said anode and said cathode is a high value and asecond state in which the impedance between said anode and said cathodeis a low value,

a power supply,

an electromagnet,

means for coupling said electromagnet in a series between one terminalof power supply and the anode `of said iirst silicon control rectiier,

a source of pulses for switching said iirst SCR from said high impedancestate Ito said low impedance state,

means for coupling said pulse source to the gate electrode of said rstSCR,

a second SCR having an anode, a cathode and a gate electrode and havinga iirst state in which the impedance from said anode to said cathode isa high value and a second state in which the impedance from said anodeto said cathode is a low lvalue,

means for coupling said one terminal of said power supply to the anodeof said second SCR,

a reference potential source,

means for coupling the gate electrode of said second SCR to saidreference potential source,

a timing capacitor coupled in series with the cathode of said secondSCR, i

vmeans for charging said timing capacitor to a potential higher thansaid reference potential,

means including said rst SCR and means for varying the time constant ofsaid timing capacitor for discharging said timing capacitor at apredetermined adjustable rate to a potential below said referencepotential, thereby causing said second SCR to switch to its lowimpedance second state, and f a commutating capacitor coupling the anodeof said rst SCR to the anode of said second SCR whereby said rst SCR isback biased and switches to its high impedance state when said secondSCR switches to its low impedance state.

2. An electronic circuit as in claim y1 wherein said time constantvarying means includes a potentiometer.

UNITED STATES PATENTS 7/1968 Piccione 317-1485 1/ 19617 McCormick.

JOHN F. COUCH, Primary Examiner. W. H. BEHA, JR., Assistant Examiner.

U.S. C1. X.R. lOl-93; 307-265, 273

